1. Field of the Invention
The present invention relates to a semiconductor process. More particularly, the present invention relates to a method for monitoring a temperature in a thermal process, especially a low-temperature thermal process.
2. Description of the Related Art
In a semiconductor fabricating process, the properties of certain parts of the devices are relatively sensitive to the temperatures in their respective forming processes, so that temperature monitor is a very important issue in those forming processes. For example, the sheet resistance of self-aligned silicide (salicide), especially nickel salicide or cobalt salicide, is very sensitive to the temperature in the thermal process for reacting the metal layer with the Si-substrate. Since the sheet resistance of the salicide layer greatly affects the device function, the temperature in the thermal silicidation process has to be monitored and controlled carefully.
For example, in a conventional cobalt salicide process, the temperature in the low-temperature thermal process for silicidation is monitored with the following steps. A cobalt layer is formed on a monitor wafer before the process, and then the monitor wafer is subject to the thermal process, together with the work wafers for forming devices, to form a cobalt silicide layer thereon. The sheet resistance of the cobalt silicide layer is measured to derive the temperature in the thermal silicidation process based on a standard temperature-sheet resistance calibration curve made previously.
However, since the monitor wafer has a cobalt silicide layer thereon after the above monitor process, it is difficult to recycle to save the production cost. Moreover, the sensitivity of the sheet resistance of cobalt silicide to the temperature in the thermal process is low in the temperature range adopted in practice, and the sheet resistance varies with the thickness of the deposited cobalt layer. Therefore, the real temperature cannot be determined precisely based on the sheet resistance. Furthermore, since the cobalt silicide is formed as a whole on the monitor wafer, local sheet resistance values cannot be determined to check the temperature uniformity in the thermal process tool.